1. Field of the Invention
The invention relates to a method for changing the section of a billet of a continuous casting plant during continuous casting operation wherein opposed sides of the billet are in operative contact with oppositely arranged roll supports positioned below a casting die, wherein the roll supports are divided into sequentially arranged segments that support rolls and are connected to one another by jointed connections, wherein each segment is independently adjustable with regard to an angle relative to the billet, and wherein in an initial position the billet guide to be changed is adjusted to a uniform production format thickness (section of the billet). The invention also relates to a device for performing the method.
2. Description of the Related Art
The change of section of a billet in continuous casting plants during the continuous casting operation is an absolute requirement for optimizing production. However, in the past it was necessary,to reduce the casting speed for a period of time for the purpose of thickness (section) reduction or thickness (section) increase. This results in production losses, and the entire course of the production is at least momentarily thrown off balance. Moreover, in the currently practiced methods it is only possible to perform a fixedly adjusted, stepped change of section.
The patent document EP 0 450 391 B1 discloses a device for supporting a metal billet, especially for soft reduction, in a strip casting plant wherein below the continuous casting die on both sides of the billet mirror-symmetrically and oppositely arranged roll supports are provided whose rolls are in operative contact with the billet. Each roll support is arranged on a stationary frame and divided into several roll-supporting segments which are connected to adjusting devices. The roll-supporting segments are connected with articulation to one another such that each segment can be independently adjusted at any desired angle relative to the billet and fixed in this position, wherein the upper adjusting device is used for the general adjustment of the roll support. This adjusting device can be a mechanical, a hydraulic or a mechanic-hydraulic adjusting device.
The patent document DE 43 38 805 C2 discloses a method and a device for operating a continuous casting plant, especially for cast-on of a continuous casting plant for producing thin slabs for hot strip rolling, with at least one reduction roll pair arranged downstream of the continuous casting die. Moveable thin slab guiding elements are positioned downstream of the reduction roll pair. The reduction roll pair is adjusted, after a predetermined length of the hot strip has passed through, to a smaller gap width which results in squeezing off of the liquid phase. The hot strip is shaped to a cast-on format having a thickness which is less than the thickness of the desired final format. The strip guiding elements, respectively, the reduction roll pair, are subsequently, preferably successively, adjusted to the thickness of the final format as soon as the cast-on format of reduced thickness is completely positioned within their adjusting area. The reduction roll pair is pressure-controlled and is positioned in accordance with the final format after the strip guiding elements have been advanced.
The patent document EP 0 743 116 A1 discloses a vertical casting production line for billets, comprising a casting die as well as a component group with foot rolls downstream of the exit of the casting die, moreover a plurality of guiding units, a correlated vertical roll segment as well as a driver arrangement in connection with a horizontal segment of the casting production line. The guiding units comprise at least the entire vertical segment of the casting production line wherein at least a portion of the rolls of the guiding elements cooperate with adjusting devices which are controlled by a process data unit in order to ensure a controllable soft reduction at least in the second part of the vertical segments.
The patent document DE 196 39 297 A1 discloses a method and a device for high-speed continuous casting plants with a billet thickness reduction during solidification. In the method and the corresponding device for continuous casting of billets whose cross-section is reduced during the solidification, wherein casting is preferably performed with an oscillating casting die, the billet cross-section is reduced linearly along a minimal length of the billet guide directly below the casting die. By means of the subsequent further billet cross-section reduction along the remaining billet guide, the so-called soft reduction, up to a point maximally directly before the final solidification or the liquid phase tip, a critical deformation of the billet can be prevented by taking into consideration the casting speed as well as the steel quality.
It is an object of the present invention to provide a method as well as a device suitable for performing the method for format thickness change (change of section) of the billet of a continuous casting plant during continuous casting, in which the casting speed for the transitional process to the change of section is not reduced, i.e., constant production and casting conditions are maintained. The transitional length of the billet during the change a section is to be shortened for the purpose of avoiding production losses. In all transitional situations of the change of section an optimal billet support is to be ensured for reducing break-out risks.
In accordance with the present invention, this is achieved in that the change of section is carried out in a controlled sequence of adjusting steps of the segments, in particular, in that
a sequential advancement of the sequentially arranged segments at their jointed connections in the casting direction is performed for reducing the format thickness or section, and
a sequential moving away of the sequentially arranged segments at their jointed connections in the casting direction is performed for increasing the format thickness or section.
With the disclosed course of the method for a change of section for continuous casting plants the following is achieved:
the casting speed is not reduced for the transition so that an increase of the production output in comparison to the prior art as well as constant production and casting conditions result;
the transition length of the billet to be produced is shortened so that production losses are reduced;
the thickness or section changes can be performed with continuously selectable values in a wide adjusting range as a function of the production program and this results in a high flexibility of the plant;
for a reduced material thickness with continuously reduced wedge shape the roll skirt provides a sufficient billet support wherein the exit side is readjusted according to the material thickness, while for format thickness increase it is achieved that with increased material thickness as a result of the increasing wedge shape the roll skirt provides a sufficient support for the billet and the exit side is readjusted according to the material thickness.
In a further embodiment of the method it is suggested that for a format thickness reduction with constant casting speed, wherein the solidification point of the billet 9 is, for example, in the segment n=3, the exit side of the first segment n=1 is advanced via the jointed connection with the inlet side of the segment n=2 by set-point control in a first adjusting phase, and, after reaching the target position, i.e., the segment position for the target format, the exit side of the segment n=2 is advanced together with the inlet side of the segment n=3 in a second adjusting phase, and in a sequence of identical steps the adjustments of the segments n=3, 4 to i to the target position is carried out.
According to another embodiment of the invention, it is proposed that for a format thickness increase at constant casting speed, wherein the liquid phase tip of the billet is, for example, located in the segment n=3, first the exit side of the segment n=1 is moved away at the jointed connection together with the inlet side of the segment n=2 by set-point control in a first adjusting phase, and, after reaching the target position, i.e., the segment position for the target format, the exit side of the segment n=2 together with the inlet side of the segment n=3 is moved away in a second adjusting phase, and in a sequence of identical steps the adjustment of the segments n=3, 4 to i to the target position is carried out.
According to a further embodiment of the method of the invention, it is proposed that the advancing of the segments is carried out with constant speed by dynamic position control, wherein a predetermined force threshold value is not surpassed.
Moreover, it is suggested according to the invention to calculate the adjusting speed of the segments by taking into consideration the permissible billet elongation limit and the current casting speed in connection with the current format adjustment, respectively, according to the resulting volume flow of the billet. Advantageously, the adjusting speed is calculated via the current casting speed, the segment length, and the required adjusting stroke according to the equation
V=Ds/Ls*Vcast
wherein Ds is the format thickness change (change of section), Ls is the segment length, and Vcast is the current casting speed.
Further developments of the method propose that the adjusting process is monitored, for example, by the current cylinder pressures of hydraulic adjusting devices and, when a threshold value is surpassed, the method switches from position control to force control and, after reaching the target position, switches back to position control.
Finally, it is suggested that the respective adjusting speeds of the exit side and the inlet side of adjoining segments are inevitably synchronous due to the jointed connection of the exit side of the segment with the inlet side of the adjoining segment.
In a device for format thickness change of the billet of a continuous casting plant, wherein opposed sides of the billet are in operative contact with oppositely arranged roll supports below the casting die, wherein the roll supports are divided into sequentially arranged segments that support rolls and are connected to one another by jointed connections, and wherein each segment is independently adjustable with regard to an angle relative to the billet, the adjusting devices are advantageously provided with means for position or force control. Expediently, the segments are in cooperative connection with controlled and direction-reversing hydraulic cylinders in the area of their jointed connections between the exit side and the inlet side.